In the course of delivery of health care services, it is often important to measure accurately the amounts of particular types of gases in the fluid in question. For example, during surgery, it is important to measure partial pressures of oxygen (pO.sub.2), carbon dioxide (pCO.sub.2), pH, and the like blood parameters, and likewise to sense some or all of such parameters in the gas mixture administered for anesthesia. Accurate monitoring of these gases, in the blood or in the anesthesia mixture, allows for accurate control or alteration of the gases administered to the patients. Similar needs often arise in intensive care units.
Most gas monitoring equipment utilizes electrochemical techniques for monitoring of gases, for example of oxygen. Most oxygen monitoring equipment involves the polarographic principle, in which an electrochemical cell is driven by a constant polarizing voltage, and the current through the cell, under proper conditions, is proportional to the amount of oxygen available to the cell. A typical oxygen electrode is shown in U.S. Pat. No. 3,826,730 to H. Watanabe et al, entitled "Disposable Electrochemical Electrode" and assigned to the assignee hereof. That patent sets forth an electrode wherein an anode and a cathode are carried in respective electrolytes and separated from the fluid being monitored by a selective gas permeable membrane. An electrical circuit is constituted by the cathode (generally a noble metal, such as gold, platinum, or silver), the electrolyte (such as saline electrolyte or potassium chloride solution), and the anode (such as silver). To the extent that oxygen is present in the fluid being monitored, it correspondingly penetrates the oxygen permeable membrane, and promotes the chemical reaction: EQU 1/2O.sub.2 +H.sub.2 O+2Ag+Cl.sup.- .fwdarw.2OH.sup.- +2AgCl.
The rate of this reaction is determined by several factors, including oxygen pressure at the cathode, cathode surface area (assuming the anode is large), membrane permeability and thickness, electrode geometry, and temperature. The factors of anode and cathode size, membrane properties, and electrode geometry are carefully controlled during the manufacturing process so that only oxygen pressure and the overall temperature affect the output of the electrode. Conventionally, temperature is compensated either by controlling the temperature of the electrode, or by monitoring the temperature and correcting the meter readout electronically.
The most common prior art approach to temperature monitoring is to place a thermistor (or a like device) in the cable connecting the electrode to the meter. In this configuration, the thermistor is placed in proximity to the liquid electrolyte or to the cathode, thereby being somewhat insulated from the room air temperature, and giving a representation of working temperature at the cathode-electrode-membrane location. It is evident, however, that such location for the thermistor cannot be totally accurate because of its spatial disparity from the reaction site. On the other hand, electrical separation must be maintained between the electrochemical cell and the temperature sensor, lest the latter have an effect on the operation of the former.